1. Field of the Invention
Embodiments of the present invention relate generally to disk drives and methods and, in specific embodiments, to a disk drive including a coarse actuator, a microactuator, and a servo controller, in which the servo controller is configured to tune microactuator feedforward values that are stored in a microactuator feedforward look-up table and to adjust a microactuator feedforward gain by which a microactuator feedforward control signal is scaled.
2. Related Art
Disk drives are widely used in computers and other electronic devices for the storage and retrieval of data. Disk drive manufacturers are constantly working to try to increase the performance of disk drives by increasing a data transfer rate of the disk drives. One main way to increase the data transfer rate of disk drives is to lower a seek time, which is a time required to position a head for a read or a write operation. Thus, disk drive manufacturers are very interested in new ways of reducing seek time.
In general, related art disk drives comprise one or more disks for storing data, a coarse actuator, a microactuator, an actuator arm assembly, one or more transducers or heads, and a servo controller. Each head is operable to read data from and to write data to concentric circular tracks on a surface of a corresponding disk. The actuator arm assembly includes a first member connected to a base of the disk drive and a second member on which the heads are mounted. The microactuator interconnects the first member and the second member of the actuator arm assembly to provide for fine positioning of the heads. The coarse actuator controls the actuator arm assembly to move the microactuator relative to the disk and, as a consequence, provides coarse positioning of the heads.
Servo controllers of disk drives typically perform the functions of seek control and track following. To perform seek control, servo controllers generally provide for a single-stage seek trajectory in which the servo controller controls the coarse actuator to move the actuator arm assembly and, as a result, position a head over a desired track of the disk for a read or a write operation. Such a servo controller may be configured to control the movement of the coarse actuator based on a coarse actuator control loop.
As part of the coarse actuator control loop, the servo controller determines a position error signal during the positioning of the head by reading servo data stored in servo sectors on the disk. The servo controller then controls the movement of the coarse actuator based on the position error signal. Some servo controllers allow for feedforward control signals to be applied to the coarse actuator control loop in order to further control the movement of the coarse actuator. The feedforward control signals for the coarse actuator control loop may be based on feedforward control values that are stored in a look-up table.
In the past, the microactuator has generally not been used for seek operations. Instead, the microactuator has typically been used for track following. Servo controllers generally perform track following once a head has been positioned over a desired track after a seek operation. Such servo controllers may control the microactuator to finely position the head with respect to the desired track based on a position error signal, so that an accuracy of track following can be increased. The range of movement of the microactuator is generally much less than the range of movement of the coarse actuator, and microactuators typically have a maximum range of movement of about a few tracks in either direction. In contrast, most coarse actuators have a range of movement that allows for movement of a head across all tracks on a disk.
However, the microactuator of related art disk drives generally provides for much faster positioning of the head than is provided for by the coarse actuator. Such a difference in positioning speed is realized because the microactuator typically comprises a piezoelectric actuator, an electromagnetic actuator, an electrostatic actuator, or the like, that moves immediately when a current is applied, whereas the coarse actuator is typically a voice coil motor (VCM) that does not move the actuator arm assembly immediately when a current is applied due to some inertia.
Recently, there has been an effort to take advantage of the rapid movement of the microactuator to reduce seek time during short seeks that seek the head over a small number of tracks. Instead of only using the coarse actuator to perform a seek operation, it has been proposed to use both the coarse actuator and the microactuator during seek operations by having a dual stage seek trajectory, including both a stepping stage and a retracting stage. Such a dual stage seek trajectory takes advantage of the ability of the microactuator to move the head rapidly within a certain range that can cover a small number of tracks.
During the stepping stage of a dual stage seek trajectory, the microactuator moves the head to a target track on the disk very quickly. Then, once the head is over the target track, the retracting stage begins in which the microactuator moves in the opposite direction of the coarse actuator so as to keep the head over the target track while the coarse actuator moves the microactuator toward the target track. In the best case, the head may be declared on-track at the end of the stepping stage when the head reaches the target track, while both the coarse actuator and the microactuator are still moving.
The microactuator must be controlled very precisely to reduce post seek track-misregistration (TMR) when a dual stage seek trajectory is employed. In order to control the microactuator, some related art disk drives have a microactuator control loop, which allows for a movement of the microactuator to be controlled in accordance with a control signal that is based on microactuator feedforward values. The microactuator feedforward values are typically precomputed and stored with the assumption that the microactuator has a linear gain. Ideally, the microactuator moves the head to the target track exactly at the end of the stepping stage and tracks the coarse actuator trajectory perfectly in the opposite direction during the retracting stage.
However, in reality, the microactuator usually does not have a perfectly linear gain, so the microactuator will not move as precisely as expected based on the control signal. Some microactuators are known to have hysteresis and nonlinearity characteristics, and their gains are known to be sensitive to temperature change. Testing shows that some microactuators have up to 20% nonlinearity in gain from small to full scale excitation, and up to 20% change in gain for a change in temperature from 0° C. to 65° C. In addition, seek TMR may also be affected by a reference track error of a coarse actuator control loop due to modeling error, and may be affected by pivot friction of the coarse actuator.
In the related art disk drives that provide feedforward control for microactuator control loops based on microactuator feedforward values that are stored in a microactuator feedforward look-up table, the microactuator feedforward values in the look-up table are typically precomputed based on a model in which a gain of the microactuator is treated as a linear gain. Also, in the related art disk drives, the microactuator feedforward values are typically precomputed and stored once, usually during the design process, and then remain stored as constant values. Since the microactuator feedforward values in the related art disk drives are precomputed with the assumption that the microactuator has a linear gain and then are stored as constant values, there is no way in the prior art disk drives to tune the microactuator feedforward values during operation of the disk drives to account for a nonlinearity of a gain of the microactuator.
Furthermore, in the prior art disk drives, a microactuator feedforward control signal is provided based on the microactuator feedforward values stored in the feedforward look-up table, and the microactuator feedforward control signal is scaled in accordance with a microactuator feedforward gain that is set and then remains constant during disk drive operation. Since the microactuator feedforward gain remains constant during disk drive operation in the related art disk drives, there is no way to adjust the microactuator feedforward gain to compensate for a change in a gain of the microactuator due to a change in temperature. Thus, the control of the microactuator will not be as precise as expected when the gain of the microactuator changes from an assumed gain that is used to set the microactuator feedforward gain to a different gain due to a change in temperature.
Thus, in the related art disk drives, there is still an undesirable amount of seek TMR associated with dual stage seek operations that inhibits on-track from being declared sooner. As a result, seek time is increased due to the undesirable seek TMR and the data transfer rate of the disk drives is reduced.
In light of the above-mentioned problems, there is a need for disk drives and methods that allow for improved control of microactuators during short seek operations with dual stage seek trajectories, so as to compensate for position errors due to modeling errors, pivot friction, nonlinearity of microactuator gains, changes in microactuator gains due to changes in temperature, and the like. Accordingly, there is a need for disk drives and methods that can provide for adaptive seek control by allowing for microactuator feedforward values to be tuned, and that can provide for microactuator gain calibration by allowing for a microactuator feedforward gain to be adjusted in real time.